Wideband radar receiver protective apparatus

ABSTRACT

A wideband radar receiver protective apparatus is provided in which a glass envelope in the shape of an elongated doughnut is utilized to provide a low pressure glow discharge in a coaxial configuration. The coaxial configuration consists of two separated sections of coaxial transmission line with the glass envelope interposed therebetween. The coaxial configuration interconnects a transmitter and the receiver to be protected. Generation of the glow plasma is controlled by means of a power source separate from the transmitter. The length of the glow region determines the amount of attenuation. The glass envelope includes an inert gas in combination with a barium getter to preserve the purity of the gas almost indefinitely.

Quinn et al.

tes Patent 1 1 July 10, 1973 WIDEBAND RADAR RECEIVER PROTECTIVEAPPARATUS [73] Assignee': The United States of America as represented bythe Secretary of the Air Force, Washington, D.C.

[22] Filed: Mar. 9, 1972 [21] Appl. No.: 233,066

[52] U.S. Cl 343/5, 333/7, 333/13, 333/99 PL, 313/220 [51] lnt. Cl. G0159/00, 1-10lp 1/14, l-l0lj 17/16 [58] Field of Search 313/220; 333/9, 13,333/99 PL; 343/5 R [56] References Cited UNITED STATES PATENTS 2,922,131l/l960 Braden 333/13 X 3,396,388 8/1968 Goldie 333/13 X 3,600,712 8/1971Williamson 333/13 X Primary ExaminerBenjamin A. Borchelt AssistantExaminerA. M. Psitos AttorneyHarry A. Herbert Jr. et a1.

[57] ABSTRACT A wideband radar receiver protective apparatus is provided in which a glass envelope in the shape of an elongated doughnut isutilized to provide a low pressure glow discharge in a coaxialconfiguration. The coaxial configuration consists of two separatedsections of coaxial transmission line with the glass envelope interposedtherebetween. The coaxial configuration interconnects a transmitter andthe receiver to be protected. Generation of the glow plasma iscontrolled by means of a power source separate from the transmitter. Thelength of the glow region determines the amount of at tenuation. Theglass envelope includes an inert gas in combination with a barium getterto preserve the purity of the gas almost indefinitely.

5 Claims, 2 Drawing Figures Pawn? SUF'FLY WIDEBAND RADAR RECEIVERPROTECTIVE APPARATUS BACKGROUND OF THE INVENTION This invention relatesto a receiver protector apparatus, and more particularly to apparatus toattenuate a signal to protect radar receivers.

Conventional transmitter-output-pulse-fired duplexers suffer from anumber of shortcomings. For example, spike leakage results in onelimitation. Spike leakage is the high power energy passing through tothe receiver during the formative time of the gas discharge.Furthermore, this formative time (e.g., 30 nanoseconds) precludesnanosecond pulses (e.g., l nanoseconds) because the transmitter pulse isshorter than the formative time. Also, bandwidth is narrow and issuitable only for long pulse narrow band radars. Fast recovery to thereceive condition is commonly achieved by adding water vapor or otherimpurities to the discharge gas; this technique makes it more difficultto initiate and maintain the discharge and shortens the life of thedevice drastically. All of these problems are eliminated by'the presentinvention.

SUMMARY OF THE INVENTION A wideband receiver protective apparatus isprovided to protect radar receivers from the high power pulses of theassociated radar transmitter. It may also be used in other similarcircumstances where it is desired to protect a vulnerable component froma de structive signal. The apparatus makes use of a low pressure glowdischarge in a coaxialconfiguration to attenuate the potentiallydestructive signal to a safe power level. Generation of the glow plasmais controlled by means of a power source separate from the transmitter;thus the apparatus is much more versatile than the conventionaltransmitter-output-pulse-fired duplexers in common use. The length ofthe glow region determines the amount of attenuation; early modelseasily attained attenuation well in excess of 100 db. The bandwidth ofthe apparatus is the bandwidth of the coaxial line, i.e., extremelybroadband, and for this reason it is ideally suited to the moresophisticated radars such as those employing pulse compression ornanosecond pulses. Recovery of the apparatus to the receive condition isaccelerated by a clean-up voltage applied to the electrodes of theappartus. The latter eliminates the need for adding water vapor or otherimpurities to the discharge gas to achieve fast recovery. Use of aninert gas such as argon in combination with a barium getter is thereforepossible thereby preserving the purity of the gas almost indefinitely(barium getters absorb nearly all gases except the inert gases) andresulting in easier faster firing and apparatus lifetime several timesthat of similar apparatus using water vapor.

The apparatus of this invention includes a discharge tube in the form ofa galss envelope mounted between two separated sections of coaxialtransmission line. The glass envelope which contains the aforementionedgas is formed of two glass cylinders of different diameters, onepositioned inside the other, and the ends of the cylinders being jointedto each other at their common ends to form a type of elongated doughnut.Operation with the gas in its dormant state achieves RF transmission. Inthe protective state, the gas is energized which sets up a lossy barrierthereby causing attenuation of the signal. The inside surface of thelarge glass cylinder and the outside surface of the smaller glasscylinder are aluminized and electrical connection made thereto. Thealuminizing does not extend to the end regions of the glass envelope toprovide windows" for passage of RF energy. The aluminized outer andinner glass cylinders in conjunction with the inner and outer conductors of the coaxial lines are utilized as blocking capacitors. In theprotective state, an appropriate voltage is applied to the aluminizedareas by way of their electrical connections to provide a uniform glowdischarge thereby presenting a very lossy medium to the RF entering oneof the windows."

The apparatus of the present invention may be used with triggerabletransmitters. The receiver protective apparatus is simply triggered intothe protective state prior to the arrival of the transmitter pulse.However, a radar system is provided that has the added advantage that itis useable with self-firing (nontriggerable) transmitters as well.

DESCRIPTION OF THE DRAWINGS FIG. 1 shows a side view of the widebandreceiver protective apparatus of the present invention; and

FIG. 2 shows a radar system including the wideband receiver protectiveapparatus.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Now referring in detailto FIG. 1, the apparatus illustrated is essentially a means for creatingand containing a glow discharge in between two separated coaxialtransmission lines 22 and 23. While there are several ways to constructthe structure the method fianlly chosen obviated the need for glass tometal seals at the two ends where RF windows are required.

Glass envelope 10 has the shape of an elongated doughnut and was formedfrom two equal length glass cylinders of the proper diameters. Smalldiameter cylinder 11 was positioned inside larger cylinder 12 and theends were bent" toward each other and sealed to form RF windows 13 and14. Barium getter section 15 is attached to the main body of glassenvelope 10. The getter is operated in the conventional manner. All ofthis construction may be performed by a skilled glass technician using aglass lathe and special torches and tools. Note that the inside surfaceof larger glass cylinder 12 and the outside surface of small glasscylinder 11 were aluminized to form aluminum metal surfaces 16 and 17,respectively. Electrical feedthrough terminals 18 and 19 were provided,prior to the glass forming, to provide electrical contacts to aluminumsurfaces 16 and 17, respectively. Further note that the aluminizing doesnot extend into the end regions so as to include RF windows" 13 and 14but extends only to end regions 20 and 21, respectively. When thestructure is complete, it is evacuated and backfrlled with argon orother appropriate gas to an approximate pressure of one mm mercury. Theexact pressure is then optimized for maximum attenuation.

Now referring again to two separated coaxial transmission lines 22 and23 which are connected by glass envelope 10. One end of glass envelope10 extends partially by a pressure fit into coaxial transmission line 22and the other end extends partially by a pressure fit into coaxialtransmission line 23. The main body of glass envelope 10 is notencompassed by either one of the coaxial transmission lines and isdotted to indicate that the relative length is usually much greater,

It is noted that coaxial transmission line 22 is comprised of innerconductor 22a and outer conductor 22b. Coaxial transmission line 23 iscomprised of inner conductor 23a and outer conductor 23b. Blockingcapacitor 24 is comprised of a portion of inner conductor 22a, a portionof glass envelope 10, and a portion of aluminized surface 17. Blockingcapacitor 25 is comprised of a portion of outer conductor 22b, a portionof glass envelope 10, and a portion of aluminized surface 16. Blockingcapacitor 26 is comprised of a portion of inner conductor 23a, a portionof glass envelope 10, and a portion of aluminized surface 17. Blockingcapacitor 27 is comprised of a portion of outer conductor 23b, a portionof glass envelope 10, and a portion of aluminized surface 16. Thus twoblocking capacitors are provided for each coaxial line.

In the nonprotective operation of this apparatus, an RF signal enters atconventional dielectric coaxial line 22. RF window 13 passes the RF withlittle loss or reflection. Immediately to the right of RF window 13 is ablocking capacitor section comprised of two blocking capacitors 24 and25, one associated with inner conductor 22a and the other with outerconductor 22b, respectively. In each case, the capacitor is formed bythe glass cylinder as dielectric sandwiched between the coaxial linecylinder and the aluminized surface on the glass. With little loss orreflection, the RF signal passes through the blocking capacitor into thealuminized glass coaxial line. The RF signal propagates down thealuminized glass coaxial line to blocking capacitors 26 and 27 which areexact duplicates of blocking capacitors 24 and 25 respectively, passingthrough to conventional air dielectric coaxial line 23. Both reflectionand loss due to the entire structure is very low for the nonprotectivestate.

In the protective state, it is desired to isolate conventional coaxialline 23 from signals entering from conventional coaxial lne 22. Toaccomplish isolation, an appropriate voltage is selectively applied toterminals 18 and 19 by power supply 28 through a predetermined valuedcurrent limiting resistor 29. The purpose of resistor 29 is to maintaina well behaved glow discharge in the volume between the inner and outeraluminized surfaces. This glow discharges presents a very lossy mediumto the RF signal, entering from coaxial line 22, thereby reducing thesignal power typically by 50 db or more. Thus, in the protective statethe apparatus has low reflection but great loss, the energy beingabsorbed in the glow discharge plasma.

When used with triggerable transmitters, the apparatus shown in FIG. 1is simply triggered into the protective state prior to the arrival ofthe transmitted pulse. However, the system illustrated in FIG. 2 has theadded advantage in that it is useable with self-firing (nontriggerable)transmitters as well.

Now referring in detail to the radar system shown in FIG. 2, receiverprotective device 30 represents the aluminized glass envelope shown inFIG. 1 in association with coaxial lines 22 and 23. Coaxial lines 22 and23 of FIG. 1 are represented as coaxial lines 32 and 33, respectively,in FIG. 2. In this embodiment, receiver protective apparatus 30 isnormally on, i.e., conventional vacuum tube or solid state switches 40and 41 are normally closed. Thus, an RF pulse emanating from transmitter31 travels through low loss (typically 3 db) attenuator 34, splits atjunction 35 with half of the energy going to antenna 36 and half beingabsorbed in receiver protective apparatus 30. It is noted power supply46 and resistor 47 are comparable to power supply 28 and currentlimiting resistor 29 respectively of FIG. 1. However, in leavingtransmitter 31 a small trigger signal is picked up conventionally fromtransmission line 37 by probe 38 and used to trigger pulser 39. Pulseroutput is a pulse of length equal to the desired radar listening time.This pulse splits, half going as pulse 44, to shut off electronic switch40. At this point in time, due to the natural delay in pulser 39 andassociated transmission lines, the RF pulse has long since left antenna36. Electronic switch 41, still being a closed switch, allows a reducedclean up voltage with amplitude determined by resistor 42 to existacross the receiver protective apparatus 30 until half of the pulserpulse delayed by delay 43, shown as pulse 45, reaches electronic switch41. Once electronic switch 41 is fully off the noise generated by theclean up current disappears. At the end of te pulse, electronic switches40 and 41 go into conduction automatically and the cycle is ready -torepeat. During the listening time, signals reflected from targets enterantenna 36 and split at junction 35, half of the energy going directlyto receiver 48 through receiver protective apparatus 30, now in the lowloss non-protective state. The other half of the energy passes throughattenuator 34 to transmitter 31 and is wasted. Any energy reflectingfrom transmitter 31 must again pass through attenuator 34 and againsplit at junction 35. Thus, assuming 3 db for the attenuator, thisnuisance signal will be at least 9 db below the direct signal to thereceiver.

It is noted that power supply 46 has two terminals 46a and 46b, and eachof electronic switches 40 and 41 has two ends and a control input. Forexample, switch 40 has ends 40a and 40b and control input 400, andswitch 41 has ends 41a and 41b and control input 410.

It is emphasized that because of the separate control feature, theapparatus may be placed in the protective state before the output pulsearrives. Still further, the use ofa barium getter to maintain the purityof an inert discharge gas such as argon is possible because water vaporor other impurities are not required to accelerate recovery as inconventional duplexers. The barium getter results in faster formativetime, lower driving re quirements and increases the life manyfold.

It is claimed: I

1. A receiver protective apparatus being comprised of an annularemission chamber formed from first and second glass cylinders ofapredetermined equal length, said first glass cylinder being of apredetermined larger diameter than said second, said second beingpositioned inside said first with the common ends bent to wards eachother and sealed to form first and second means for passage of RFsignals therethrough, a first aluminized surface applied to the innersurface of said first glass cylinder excluding said first and secondpassage means, a second aluminized surface applied to the outsidesurface of said second glass cylinder excluding said first and secondpassage means, an inert gas at a predetermined pressure contained insaid annular chamber, a barium getter integrated with said annularchamber, receiver means, first and second coaxial transmission linesphysically separated from each other, said first coaxial transmissionline receiving RF signals, said second coaxial transmission line feedingRF signals to said receiver, said annular emission chamberinterconnecting said first and second coaxial transmission lines with aportion of said annular emission chamber including said first passagemeans being inserted into said first coaxial transmission line and aportion of said annular emission chamber also including said secondpassage means being inserted into said second coaxial transmission line,current limiting means, and power supply means to selectively apply apreselected voltage through said current limiting means to said firstand second aluminized surfaces to provide a glow discharge therebetweenthus operating to provide a preselected attenuation of the RF signalsbeing fed to said reciever.

2'; A receiver protective apparatus *as' described in claim 1 whereinsaid inert gas is comprised of argon.

3. A receiver protective apparatus as described in claim ll wherein saidcurrent limiting means is comprised of a resistor.

41. A radar system including a receiver protective apparatus beingcomprised of transmitter means providing RF output pulses to betransmitted towards targets of interest, antenna means directing said RFoutput pulses towards said targets, a first coaxial transmision lineconnected to said antenna means, receiver means, a second coaxialtransmission line connected to said receiver, said first and secondcoaxial transmission line being physically separated, an annularemission chamber formed from first and second galss cylinders of apredetermined equal length, said first glass cylinder being of apredetermined larger diameter than said second, said second beingpositioned inside said first with the common ends bent towards eachother and sealed to form first and second passage means for RF signalstherethrough, a first aluminized surface applied to the inner surface ofsaid first glass cylinder exclusing said first and second passage means,a second aluminized surface applied to the outer surface of said secondglass cylinder excluding said first and second passage means, an inertgas at a predetermined pressure contained in said annular emissionchamber, a barium getter integrated with said annular emission chamber,said annular emission chamber electrically interconnecting said firstand second coaxial transmission line at said physical separationtherebetween with a portion of said annular emission chamber includingsaid first passage means inserted in said first coaxial transmissionline and with a portion of said annular chamber including said secondpassage means inserted in said second coaxial transmission line, powersupply voltage means having first and second terminals, said firstterminal being connected to said first aluminized surface, first andsecond switches normally closed, each of said switches having first andsecond ends and a control input, said second terminal being connected tosaid first ends of said first and seond switches, current limiting meansconnected between said second end of said first electronic switch andsaid second aluminized surface to provide a glow discharge between saidfirst and second aluminized surfaces during the closed period of saidfirst electronic switch, resistor means connected from said second endof said second electronic switch and said second end of said firstelectronic switch to provide a reduced voltage to exist across saidannular emission chamber during the open period of said first electronicswitch and the closed period of said second switch, means to couple atrigger pulse from said RF output pulses, pulse means providing acontrol pulse of predetermined length in response to receipt of saidtrigger pulse, said predetermined length being determined by thelistening period of said radar system, said control input of said firstelectronic switch receiving said control pulse and operating the opensaid first electronic switch, and means for a predetermined delaypassing said control pulse to said control input of said secondelectronic switch also for opening thereof, said first and secondswitches automatically closing at the end of said control pulse and thedelayed control pulse.

5. A radar system as defined in claim 4 further including a preselectedattenuator interposed between said transmitter means and said antennameans.

1. A receiver protective apparatus being comprised of an annularemission chamber formed from first and second glass cylinders of apredetermined equal length, said first glass cylinder being of apredetermined larger diameter than said second, said second beingpositioned inside said first with the common ends bent towards eachother and sealed to form first and second means for passage of RFsignals therethrough, a first aluminized surface applied to the innersurface of said first glass cylinder excluding said first and secondpassage means, a second aluminized surface applied to the outsidesurface of said second glass cylinder excluding said first and secondpassage means, an inert gas at a predetermined pressure contained insaid annular chamber, a barium getter integrated with said annularchamber, receiver means, first and second coaxial transmission linesphysically separated from each other, said first coaxial transmissionline receiving RF signals, said second coaxial transmission line feedingRF signals to said receiver, said annular emission chamberinterconnecting said first and second coaxial transmission lines with aportion of said annular emission chamber including said first passagemeans being inserted into said first coaxial transmission line and aportion of said annular emission chamber also including said secondpassage means being inserted into said second coaxial transmission line,current limiting means, and power supply means to selectively apply apreselected voltage through said current limiTing means to said firstand second aluminized surfaces to provide a glow discharge therebetweenthus operating to provide a preselected attenuation of the RF signalsbeing fed to said receiver.
 2. A receiver protective apparatus asdescribed in claim 1 wherein said inert gas is comprised of argon.
 3. Areceiver protective apparatus as described in claim 1 wherein saidcurrent limiting means is comprised of a resistor.
 4. A radar systemincluding a receiver protective apparatus being comprised of transmittermeans providing RF output pulses to be transmitted towards targets ofinterest, antenna means directing said RF output pulses towards saidtargets, a first coaxial transmision line connected to said antennameans, receiver means, a second coaxial transmission line connected tosaid receiver, said first and second coaxial transmission line beingphysically separated, an annular emission chamber formed from first andsecond glass cylinders of a predetermined equal length, said first glasscylinder being of a predetermined larger diameter than said second, saidsecond being positioned inside said first with the common ends benttowards each other and sealed to form first and second passage means forRF signals therethrough, a first aluminized surface applied to the innersurface of said first glass cylinder excluding said first and secondpassage means, a second aluminized surface applied to the outer surfaceof said second glass cylinder excluding said first and second passagemeans, an inert gas at a predetermined pressure contained in saidannular emission chamber, a barium getter integrated with said annularemission chamber, said annular emission chamber electricallyinterconnecting said first and second coaxial transmission line at saidphysical separation therebetween with a portion of said annular emissionchamber including said first passage means inserted in said firstcoaxial transmission line and with a portion of said annular chamberincluding said second passage means inserted in said second coaxialtransmission line, power supply voltage means having first and secondterminals, said first terminal being connected to said first aluminizedsurface, first and second switches normally closed, each of saidswitches having first and second ends and a control input, said secondterminal being connected to said first ends of said first and secondswitches, current limiting means connected between said second end ofsaid first electronic switch and said second aluminized surface toprovide a glow discharge between said first and second aluminizedsurfaces during the closed period of said first electronic switch,resistor means connected from said second end of said second electronicswitch and said second end of said first electronic switch to provide areduced voltage to exist across said annular emission chamber during theopen period of said first electronic switch and the closed period ofsaid second switch, means to couple a trigger pulse from said RF outputpulses, pulse means providing a control pulse of predetermined length inresponse to receipt of said trigger pulse, said predetermined lengthbeing determined by the listening period of said radar system, saidcontrol input of said first electronic switch receiving said controlpulse and operating the open said first electronic switch, and means fora predetermined delay passing said control pulse to said control inputof said second electronic switch also for opening thereof, said firstand second switches automatically closing at the end of said controlpulse and the delayed control pulse.
 5. A radar system as defined inclaim 4 further including a preselected attenuator interposed betweensaid transmitter means and said antenna means.